This invention relates to systems for controlling the diameter of a crystal while growing in a crystal growing furnace, and, while the invention is subject to a wide range of applications, a preferred embodiment of the invention will be particularly described as applied to a system response to tracking angular movement of a spot of light reflected from a meniscus for governing the growth of the crystal. In the Czochralski puller method of crystal growth operation begins with semi-conductor grade polysilicon chunks that are placed in a fused quartz crucible. The crucible is set inside a chamber and is surrounded in the chamber by a heater for melting the polysilicon in a nonreactive atmosphere, and maintaining the temperature of the melt at just above the freezing point. A seed crystal is dipped into the melt and is thereafter slowly raised from the melt as it is rotated in one direction and the melt is rotated in the opposite direction so that the liquid from the melt will adhere to the withdrawing seed crystal and cause an elongated crystal rod to be grown. The diameter of the crystal grows at a rate governed mainly by the rate of lift of the seed relative to the melt and the melt temperature. After the crystal has grown initially to the desired diameter for the crystal, it is desirable that the diameter be closely maintained by regulating the pulling rate, temperature, and the like.
In this method of growing crystals, the seed is vertically suspended by a cable connected to lift and rotation drive apparatus that can be closely regulated, and wherein the crystal is substantially free of vibration from the lift and rotation mechanism. There are, however, conditions where the crystal does not rotate in a perfect circle, and it can move in an orbit, for example, which has a tendency to cause undesirable diameter changes in some systems for sensing the diameter by light reflected from a meniscus, and, in some cases, to completely lose automatic closed loop control of crystal diameter.
An object of the present invention is to provide an improved system for controlling the diameter of a crystal which substantially obviates one or more of the limitations and disadvantages of the described prior art systems.
Other objects, purposes and characteristic features of the present invention will be in part obvious from the accomanying drawings, and in part pointed out as the description progresses.